The present invention generally is directed to methods for removing a charge on a specimen. More particularly, the invention is directed to a method for electrically discharging a specimen positioned within a vacuum chamber of an electron scanning device.
There is an increasing need for scanning electron microscopes in all areas of the development and manufacture of microelectronic and optoelectronic components in order to be able to visually evaluate structures on a sub-micrometer scale thereby to identify deviations from specified patterns and in order to be able to acquire and evaluate topographical parameters such as heights, widths or angles of inclination. Conventional scanning electron microscopes, however, do not have the required spatial resolution of fractions of a micrometer through a few nanometers at voltages below approximately 20 kV. However, at voltages above about 20 kV, resist structures and intergrated circuits are damaged due to the high-energy electrons. Further, non-conductive specimens become charged.
In order to produce an adequately high surface or, respectively, volume conductivity so that a non-conductivity specimen can be observed in a scanning electron microscope, non-conductive specimens are usually vapor-deposited with a thin metal layer to thereby be provided with a conductive surface. A metalization of the surface, however, cannot be used when the specimen is to be re-employed or is to be subjected to further treatment after its examination in the scanning electron microscope.
In order to also be able to image and investigate non-metalizable specimens in a charge-neutral manner, an attempt has been made to match the energy of the electrons to what is referred to as the neutral point energy of the specimen material. Since the neutral point energy is generally not exactly known, local chargings that influence the electron beam and diminish the spatial resolution occur over and over again.
In IBM Technical Disclosure Bulletin, Volume 27, No. 11 (1985), pages 6388-6389 fully incorporated herein by reference, there is disclosed a method for discharging a semiconductor wafer situated in an electron beam system. The elimination of the charges applied by the electron probe ensues therein via a photoconductive layer that is produced by irradiating the wafer with ultra violet light.
It is also known to remove the charge present on a specimen with the assistance of an electron beam or ion beam directed onto the specimen surface as discussed, for example, U.S. Pat. No. 4,249,077, fully incorporated herein by reference, and Nuclear Instruments and Methods 149, 1978, pages 587-590, also fully incorporated herein by reference. In the method disclosed in J. Phys. D: Applied Physics, Volume 11, 1978, pages 2315-2325, fully incorporated herein by reference, nitrogen ions which serve to neutralize a specimen charge are generated in an electron surge in an electrical field built up above the specimen.